Published online 27 May 2010 | Nature | doi:10.1038/news.2010.267

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Ardi may be more ape than human

Woodland home and hominid ancestry of Ardipithecus ramidus questioned.

Ardi Science coverDoubts have been raised over Ardi's reported classification and habitat.T. White

A fight has broken out over attempts to drag 'Ardi' – the oldest hominid skeleton found – out of the woods where her discoverers say she lived.

And there are new questions about the classification of Ardipithecus ramidus aka Ardi, dated at 4.4 million years old; whether the species found in the Rift Valley of the Afar of Ethiopia is an ape or hominid.

The debate is playing out this week in Science with the publication of two technical comments1,2. One focuses on isotopic data from soils, plants and animals suggesting that the species' habitat was likely to have been a more open savannah with some trees rather than woods; the second contends that A. ramidus is more ape-like than human.

Tim White of the University of California at Berkeley and colleagues, who last October published 11 articles in Science on Ardi (see 'Fossil rewrites early human evolution'), insist in two responses3,4 that their reports properly place what is clearly a hominid in its preferred woodland habitat.

The argument isn't an esoteric dispute among palaeontologists, but speaks to a fundamental theory of human evolution: that hominids began walking upright in response to the spread of grasslands in eastern Africa less than 8 million years ago.

In the special Science issue on Ardi last October, the articles largely undermined that view, which dates to Raymond Dart's theories in the 1920s. The papers from White and colleagues showed a bipedal species with ape-like feet and arms living in woodlands, but not knuckle-walking like a chimpanzee.

While the opposing points of view are being vigorously argued this week in Science, observers say a final answer to the question remains distant.

"This is not a simple issue to answer," says Matt Sponheimer, an archaeometrist at the University of Colorado in Boulder who peer reviewed the isotopic comment for Science. "We don't want to throw the savannah hypothesis out prematurely. We need broader samples over a longer period of time."

Open country

In their technical comment, Thure Cerling of the University of Utah in Salt Lake City and colleagues argue Ardi didn't live in the "closed wooded habitat" proposed originally by White and colleagues, but that the specimens came from a strip of trees along a waterway through a savannah1.

They base this interpretation on an analysis of stable carbon isotopes in preserved soil, or 'palaeosol', at the site; oxygen and carbon isotopes in the enamel of mammalian teeth; the small-mammal fossils present; and the types of silica-rich plant remnants called phytoliths.

The Cerling group contends that the abundance of small-mammal fossils at the site — which White's group says supports a woodland environment — could be due to predators hiding in vegetation growing around water and ambushing animals coming to drink.

In an interview, White accused the critics of "inaccurate" presentations created by "cherry picking data" — while overlooking key factors among more than 6,000 vertebrate specimens he and his colleagues catalogued.

For instance, White notes that isotopic analysis of tooth enamel of Ardi herself shows a diet from a woodland habitat.

Cerling denies misusing data. The Ardi tooth-enamel isotopic results were not discussed due to restrictions on article length, he says. But, he adds, the tooth-enamel findings do not directly support the idea that Ardi lived in woodland.

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In the other technical comment in Science, primatologist Esteban Sarmiento says he questions whether Ardi is in the human lineage because the fossil probably predates the divergence between humans and apes, which he estimates as 3 to 5 million years ago2.

Ardi's age is so close to that divergence date that no unequivocal determination can be made about whether she is in the ape or human lineage, says Sarmiento, who conducts research from home in East Brunswick, New Jersey.

But White and co-authors disagree. In their response4, the group says Sarmiento's "tortuous, nonparsimonius evolutionary pathways" are not supported by many of the fossil's characteristics. 

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